Shift registers have been formed in a variety of architectures. Conventionally, these architectures have been implemented with transistors. For example, one common implementation uses a D flip-flop that includes master and slave components of cross-coupled depletion-load NOR gates, with each component including six transistors. However, there are several disadvantages associated with using transistors to implement a shift register.
First of all, devices used in digital circuits are becoming smaller and smaller. As these devices decrease in size, quantum mechanical effects begin to appear. The electrical properties of conventional transistors may be altered in an unacceptable way by these quantum mechanical effects. Secondly, a transistor-implemented shift register is limited by the switching speed of the transistors, which may be too slow for some applications.